The invention relates to the continuous casting of metals. More specifically, it relates to electromagnetic devices fitted into continuous casting molds and acting on the liquid metal present in said molds.
The use of electromagnetic fields to have an influence on the movements of the liquid steel in continuous casting molds of any format is in practice at the present time. The main objectives of imposing rotating electromagnetic fields (in the case of casting blooms and billets of square or slightly rectangular cross section) or traveling electromagnetic fields (in the case of casting slabs of rectangular cross section, the width of which is much larger than the thickness) are to homogenize the solidifying structures over the entire cross section of the product and to improve the surface finish of the product, together with its cleanliness from the standpoint of inclusions, especially near its surface. When casting slabs continuously, it is also known to impose static electromagnetic fields in the mold in order to stabilize the meniscus (i.e. the free surface of the molten metal in the top of the mold). This stabilization makes it possible to increase the product casting rate and therefore the productivity of the continuous caster. The electromagnetic devices producing this effect are known as xe2x80x9celectromagnetic brakesxe2x80x9d.
The known uses of electromagnetic fields in continuous casting molds have, for the moment, not been sufficient to solve completely satisfactorily all the problems of cast product quality. Among these persistent problems, mention may be made of the following:
the improvement in surface quality of the as-cast products, which corresponds to the reduction in the number of surface cracks and in the depth of oscillation ripples;
the improvement in the subshell cleanliness of the cast product, which corresponds to a reduction in the size of the xe2x80x9csolidification hooksxe2x80x9d which form during oscillation of the mold, these hooks being potential sites for the trapping of inclusions and gas bubbles present within the liquid metal in the mold, and also to the elimination of inclusion pickup by the solidification front, benefiting from the effect of this front being xe2x80x9cwashedxe2x80x9d by the liquid metal entrained by the electromagnetic stirring (the mechanisms relating to these problems will be described in detail below);
the achievement of meniscus stability sufficient to guarantee optimum lubrication of the mold/solid metal interface by the covering slag which, in the liquid state, infiltrates therein so that this improved lubrication results in casting rates significantly greater than the usual rates.
Solving these problems satisfactorily would result in an increase in productivity of the caster and of the entire steel works. In addition to the increase in casting rate already mentioned, it would reduce the frequency of crack removal operations (in which the surface of the product is ground in order to eliminate defects therein) and would thus increase the proportion of products having a sufficient quality to be sent directly to the hot rolling mill. However, no currently known technique allows all the aforementioned qualitative objectives to be met simultaneously in an optimum fashion. In addition, the known techniques for achieving one or other of the these objectives are either expensive or require tricky implementation as they are very sensitive to the other casting conditions. Among these, apart from the methods mentioned above involving magnetic fields, mention may be made of systems which apply nonsinusoidal oscillations on the mold, embossed molds having a controlled hot-face roughness, covering slags of optimized composition, etc.
The object of the invention is to provide a process and a plant for the continuous casting of metals, which meet the productivity and quality objectives expected by operators of casters for continuously casting metals, especially steel.
With these objectives in mind, the subject of the invention is a process for the vertical continuous casting of metal products in a mold having cooled plates joined together, in which process the region of the meniscus of the liquid metal present in the mold is subjected to the action of an axial alternating magnetic field, collinear with the direction of casting, tending to impose on said meniscus a domed overall shape, characterized in that said region of the meniscus is also subjected to a continuous magnetic field directed transversely to the direction of casting in order to allow the shape of said meniscus to be stabilized.
The subject of the invention is also a plant for the vertical continuous casting of metals, comprising a mold having cooled plane plates joined together, of which two are long, facing one another in order to define a casting space, which plant is of the type having an electromagnetic coil supplied with AC current and surrounding the mold in the region of the meniscus of the liquid metal which is present therein so as to produce therein an alternating magnetic field directed along the casting axis, characterized in that it also includes an electromagnetic inductor which produces a continuous magnetic field passing through the long plates of the mold in the region of the meniscus perpendicular to the casting axis .
As will have been understood, the invention consists in creating at least two electromagnetic fields in the liquid metal present within the continuous casting mold, these fields acting simultaneously on said metal in the region of the meniscus. On of these fields is an axial alternating field and the other is a transverse continuous field, both being exerted in the region of the meniscus. They are produced by means of fitted inductors or inductors producing their effect near the meniscus.
Schematically speaking, the alternating field collinear with the casting axis is used to xe2x80x9cdomexe2x80x9d the meniscus, that is to say to define the convex dome shape that it naturally assumes on contact with the walls of the mold, while the transverse continuous field acts as an electromagnetic brake in order to reduce the local geometrical irregularities at the surface of this meniscus, resulting in subjacent convection currents generated by this alternating field.
Theoretically, applying a single alternating magnetic field might suffice by itself to obtain a smooth domed meniscus. This is because the electromagnetic force generated on the liquid metal has both:
a confining surface component which tends to push the periphery of the meniscus away from the sides of the mold, and therefore to xe2x80x9chollowxe2x80x9d it around the border, smoothing out its surface. This force is especially active at high frequency; and
a stirring volume component which, because of the configuration of the convective currents in the liquid metal that it causes (annular stirring with the metal rising in the center of the mold), xe2x80x9cswellsxe2x80x9d the central part of the meniscus. This force is, in contrast, especially active at low or medium frequency. Moreover, it is for this reason that it is the cause of surface instabilities. The maximum effect of this stirring force is obtained at a medium frequency, namely around 200 Hz to be specific, but in any case less than 500 Hz, whatever the nature or the thickness of the mold or the format of the metallurgical product cast.
It is these two conjugate actionsxe2x80x94peripheral repulsion and stirring with central rising (which actions could be obtained from one and the same pulsating magnetic fieldxe2x80x94which give the meniscus a desired defined domed shape.
By the same token, but for the purpose of solidifying the electromagnetically confined metal, that is to say metal away from any physical contact with the cooled sides of the mold, it has already been proposed to create a magnetic environment within the mold, consisting of the superposition of two axial fields, that is to say both fields being directed along the casting axis, one being periodic (the confining field) and the other being constant in order to produce radial vibration forces in the confined liquid metal. These fields are generated by individual coils around the top of the mold, one being supplied with AC current at a frequency of between 500 and 5000 Hz, the other being supplied with DC current. To limit the stirring effect of the alternating field, it has either been proposed to add a third surrounding coil in order to create, at the point where the two previous fields already act, an additional periodic axial magnetic field at power frequency (EP-A-0 100 289 or the article by Ch. Virves, xe2x80x9cEffects of forced electromagnetic vibrations during the solidification of aluminium alloys: Part II. Solidification in the presence of collinear variable and stationary magnetic fieldsxe2x80x9d published in the journal Metallurgical and Materials Transactions B, Vol. 27B, No. 3, Jun. 1st, 1996, pages 457 to 464). Again this type of teaching is found, for example, very briefly in this case, in document DE 35 17 733 (1986), which also proposes the use, beside a high-frequency variable axial confining magnetic field, of a continuous field which may be either axial or transverse, but which has to act over the entire height of the mold, thereby inevitably resulting in electromagnetic arrangements of extreme complexity from the technological standpoint.
That said, whatever the intended applicationxe2x80x94solidification with confinement or, like the present invention, geometrical control of the meniscusxe2x80x94the problem that arises is to be able to transfer enough electromagnetic energy through the copper mold into the cast metal. At the frequency levels adopted (greater than 500 Hz), it would in fact be necessary, because of the magnetic screening effect that the metal walls of the mold present, to segment it vertically in order to allow it to behave like an xe2x80x9celectromagnetic cold cruciblexe2x80x9d.
Such an arrangement is complex to put into practice both from the electromagnetic standpoint, because of the inevitable electrodynamic instabilities associated with the liquid nature of the final armature (the liquid metal within the mold) which is acted upon by the intermediate susceptor that the mold itself is, and also by the facte that the mold is above all a bottomless vertical crystallizer, the lateral sealing of which must always be perfect, the format of which must be geometrically stable (to avoid bulging phenomena in the long walls) and the cooling circuit of which is strictly optimized. Such a segmentation of the mold, in particular of the long side walls, would require having to completely reconsider an already proven design of the mold from the technological standpoint and from the functional standpoint.
In fact, because of its construction based on four copper or copper alloy plates joined together in the corners (two facing plane long walls and two short end walls), a slab mold naturally acts like a xe2x80x9ccold cruciblexe2x80x9d, but for moderate frequencies. At 200 Hz, most of the electromagnetic power delivered by an inductor can be transferred without any difficulty into the molten metal through the walls, the thickness of which rarely exceeds 40 or 45 mm. However, at this frequency, the meniscus deformation resulting, as explained above, from the combination of the confining force and the convection of the metal, results in large fluctuations over time in the xe2x80x9cmeanxe2x80x9d shape of the meniscus. This is why, according to an essential feature of the invention, a continuous magnetic field is applied, directed perpendicular to the casting axis, which field, also applied in the meniscus region, will act as an electromagnetic brake on the subjacent liquid-metal convection currents generated by the centripetal force at 200 Hz doming the meniscus and will consequently have a smoothing effect on the meniscus surface.